It is possible to recognize the internal state of a living body or a substance by irradiating the living body or the substance with X-rays and detecting and visualizing transmitted X-rays on the basis of the transmitted X-ray imaging technique. In the transmitted X-ray imaging technique, a photographic plate or a photograph film is used to detect the transmitted X-rays. In recent years, however, development of an X-ray flat panel detector has been advanced energetically (see, for example, Japanese Patent Application Laid-Open No. 2010-098621). As for the X-ray flat panel detector, there are a direct conversion system in which X-rays are converted directly to an electric signal and an indirect conversion system in which X-rays are converted to an optical signal and then converted to an electric signal. In either of the systems, filmless imaging can be implemented and it becomes possible to conduct picture quality improvement and diagnosis supporting. Furthermore, there is an advantage that, for example, electronic filing and networking are facilitated, and utilization in various fields is expected.
An equivalent circuit diagram of an example of imaging elements and a current/voltage conversion circuit included in an X-ray flat panel detector is shown in FIG. 50. Here, a plurality of (M×N) imaging elements 230 arranged in X and Y directions in a two-dimensional matrix form convert incident X-rays to a current directly (direct conversion system) or indirectly (indirect conversion system). A plurality of (M) imaging elements 230 arranged in the X direction are connected to one current/voltage conversion circuit 240 via switch circuits 232 and a row wiring line 233. The current/voltage conversion circuit 240 converts a current from each imaging element to a voltage in order. In FIG. 50, reference numeral 231 denotes parasitic capacitance (capacitance value: Cpd) the imaging element 230 has. The current/voltage conversion circuit 240 is a well-known current/voltage conversion circuit (a kind of integral circuit) including an operational amplifier 241, a capacitor section 242 (capacitance value Cint), and a short-circuiting circuit 243 having a reset switch circuit 244. A reference voltage VRef is input to a non-inverting input section of the operational amplifier 241. Furthermore, an inverting input section of the operational amplifier 241 is connected to the row wiring line 233. The capacitor section 242 and the short-circuiting circuit 243 are connected in parallel, and connected to an inverting input section and an output section of the operational amplifier 241.
When starting the current/voltage conversion circuit 240, the switch circuit 232 is brought into an off state and the reset switch circuit 244 is brought into an on state, and thereby potential across the capacitor section 242 is set equal to VRef. Then, the reset switch circuit 244 is brought into an off state, and X-rays are incident on the imaging element 230. The imaging element 230 converts an incident electromagnetic wave to a current. This current is stored in the parasitic capacitance 231 as charge Qin. When the switch circuit 232 is brought into an on state, the charge Qin stored in the parasitic capacitance 231 is transferred to the capacitor section 242 in the current/voltage conversion circuit 240 via the row wiring line 233. Denoting an output voltage which is output from the current/voltage conversion circuit 240 by V0, finally V0=Qin/Cint 
is obtained.